No storyline details were revealed, though that could change later today as assorted #DCTV EPs appear at TCA panels.

Explaining the scheduling change-up/special Monday Arrow episode, CW boss Mark Pedowitz said that with The Flash and Legends already paired up on Tuesdays — and after discussions with producers and the marketing department — “We felt that it would be better and tighter in storytelling…. This was the best way to go.”

“Next year we may go back to four nights,” he allowed.

Pedowitz also made clear that unlike last year’s event, which technically started but barely took place on Supergirl, “It’s a full, four-episode crossover.” As for how one tops an alien invasion as a premise for the all-star team-up, all he would say is, “Romance is in the air.”

The Arrowverse’s December 2014 crossover event took place on The Flash and then Arrow. In December 2015, Legends of Tomorrow villain Vandal Savage targeting Central City barista Kendra Saunders, triggering the involvement of Flash and Arrow heroes.

Last year’s four-show, three-night event folded in Supergirl, as the many Superfriends battled the alien race known as The Dominators.